Excitons charge-transfer

Loi MA, Toffanin S, Muccini M, Forster M, Scherf U, Scharber M (2007) Charge transfer excitons in bulk heterojunctions of a polyfluorene copolymer and a fullerene derivative. Adv Funct Mater 17 2111... 

Veldman D, Ipek O, Meskers SCJ, Sweelssen J, Koetse MM, Veenstra SC, Kroon JM, van Bavel SS, Loos J, Janssen RAJ (2008) Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends. J Am Chem Soc 130 7721... 

A theoretical study by the same author suggests that RDX forms charge transfer complexes upon crystn which are unique because their charge transfer exciton band is of lower energy than the singlet exciton band of their molecular crysts. Static reactivity indices were used to predict the likely primary dissociative products obtainable from each excited state of secondary nitramines theory predicts that the axial and equatorial nitramine groups of the polynitra-mines RDX, alpha- and beta-HMX, may possess quite novel selective decompn paths and hence give different primary dissociative products... 

Poly(phenylmethylsilane) (PMPS) was investigated by Kepler et al. (1983, 1984, 1987). Figure 32 shows the wavelength dependence of the absorption and the normalized photocurrent. The spectral dependence of the photocurrent indicates that carrier generation occurs by an extrinsic process which results primarily from an interaction of charge-transfer excitons with the surface. The argument is similar to that proposed earlier by Kepler (1976) to describe hole... 

Electro-absorption (EA) spectroscopy, where optical absorption is observed under the application of an electric field to the sample, is another method that can distinguish between localised and inter-band excitations. The electric field produces a Stark shift of allowed optical absorptions and renders forbidden transitions allowed by mixing the wavefunctions of the excited states. Excitons show a quadratic Stark (Kerr) effect with a spectral profile that is the first derivative of the absorption spectrum for localised (Frenkel) excitons and the second derivative for charge transfer excitons, i.e. 

Electroabsorption of Azo-TPA (Scheme 5) is interpreted in terms of a charge transfer exciton rather than a Frenkel exciton as formed by Azo-FO [341]. From this and other studies it is suggested that carrier generation from charge transfer excitons is more efficient than from Frenkel excitons [34m]. 

The soliton concept is likely to prove fruitful in many aspects of photochemistry especially where organized systems are involved, It has been proposed that charge transfer excitons in mixed-stack donor-acceptor compounds exist in a variety of solitonic states . In this effect charge transfer states couple to lattice phonons in the crystal lattice to form the comparatively stable solitons. 

Charge transfer from the 11 Bu to an adjacent molecule or segment of a chain, i.e., dissociation of the 11 Bu. This process may also be extremely fast.24 Indeed, so fast that it has been suspected that this charge transfer state, aka a spatially indirect exciton, charge transfer exciton (CTE), or intermolec-ular or interchain polaron pair, may be generated directly from the ground state.24... 

The PL quantum yield r)pl. While r]pl of many dyes is close to 100% in solution, in almost all cases that yields drops precipitously as the concentration of the dye increases. This well-known concentration quenching effect is due to the creation of nonradiative decay paths in concentrated solutions and in solid-state. These include nonradiative torsional quenching of the SE,148 fission of SEs to TEs in the case of rubrene (see Sec. 1.2 above), or dissociation of SEs to charge transfer excitons (CTEs), i.e., intermolecular polaron pairs, in most of the luminescent polymers and many small molecular films,20 24 29 32 or other nonradiative quenching of SEs by polarons or trapped charges.25,29 31 32 In view of these numerous nonradiative decay paths, the synthesis of films in which r]PL exceeds 20%, such as in some PPVs,149 exceeds 30%, as in some films of m-LPPP,85 and may be as high as 60%, as in diphenyl substituted polyacetylenes,95 96 is impressive. 

The intense visible alssorption of PDA peaking at 600-650 nm has been interpreted using a phenomenological charge-transfer exciton model (29). A more fundamental understanding is desirable, especiedly as a basis for interpretation of third-order nonlinear optictil phenomena, and such research has been initiated. 

Linearity of the above relation allows one to find the tensor (01, k) by calculating the polarization P, induced in the crystal by the total transverse field E4-, neglecting the local counterpart of the transverse field which is very small.48 As unperturbed states in this case we have to use the Coulomb exciton states which are obtained taking full account of the Coulomb interaction between charges. Importantly in this case we can assume that charge transfer excitons are also taken into account. If the unperturbed states, obtained by taking into account the full Coulomb interaction, are known, considering the field E4- as a perturbation we can determine the polarization... 

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